Just got this in the mail - it was supposed to be 4 books, but all I got was the wadded up envelope in a PO baggie   It looks like plenty of packing tape was used, but a machine shredded it all up.

At least the sender used PBS postage, so they still get the credits.  I marked it 'received-empty envelope', so it looks like I wait until it times out.

PBS Help Center says to submit a request to the PO to look for the contents, but that seems like a big waste of time - 4 books with no identifying information - how would they possibly know where they belong?

Yeah, sorry, the odds are not in your favor :/ 

The key with shipping books is that whatever it is being sent in has to be TIGHT around it. If the books can move at all, the odds of the machines damaging the packaging are much higher. If they are in a box, all empty spaces should be filled so that they cannot move. 

I am going to fill out the form tomorrow and will keep you posted as I hope they get found, because this was one of the sets that the post office hadn't scanned (see my post above).  I am not asking for credits, just letting you know the status on the form.  
Theresa